1. Field of the Invention
The present invention is related to a control circuit of a DC fan motor for start with high voltage and high rotational speed with low voltage and particularly to a control circuit preventing the control element of a motor from damage.
2. Brief Description of the Related Art
Due to highly developed information technology in our society, it is a required function of the electronic product that a great deal of electronic data can be transmitted and treated effectively. However, a side effect of occurring a phenomenon of generating high temperature always comes with the function. Hence, it is necessary for the electronic product to provide a heat dissipation apparatus with excellent performance and needing less power in order to solve the problem of heat dissipation.
Currently, the cooling fan and the radiator are heat dissipation apparatuses available on the market. The former helps to dissipate heat by way of convection and the latter helps to dissipate heat by way of conduction. Because forced heat dissipation created with the cooling fan provides better effect, the cooling fan is kept being developed by all suppliers unceasingly.
Referring to FIG. 1, a conventional circuit of a fan motor includes a motor 11 and a driving unit 12. The motor 11 includes at least a first coil set 111 and a second coil set 112. The driving unit 12 is connected to the motor 11 and at least includes a driving element 121 and a Hall element 122. The Hall element 122 senses magnetic pole changes of a rotor in the motor 11 to generate positive voltage H+ and negative voltage H− outputting to corresponding pins in the driving element 121. The driving element 121 has a first output pin 1211 connecting with the first coil set 111 and has a second output pin 1212 connecting with the second coil set 112 for actuating the motor 11 to run with a proper rotational speed.
However, the preceding conventional circuit has problems in practice. Once the conventional circuit has been made completely in accordance with the original design, performance of the motor 11 is unchangeable except redesign. That is, the input voltage has a proportional relation to the rotational speed of the motor as the T1 curve shown in FIG. 2. For instance, when the motor 11 starts with the input voltage of 12 volts, the rotational speed of the DC fan motor 11 is 4,000 rpm and when the motor 11 runs with the input voltage of 4 volts, the rotational speed of the DC fan motor is 1,000 rpm. Under this circumstance, the rotational speed with the low input voltage becomes a little too low and it is not possible to attain the required efficiency of heat dissipation.
In order to increase the rotational speed with the same low input voltage, the coil sets were redesigned to allow the DC fan motor 11 provides a rotational speed of 2,300 rpm with input voltage of 4 volts as T2 curve shown in FIG. 2. However, the high input voltage leads to increased rotational speed as well, i.e., the high input voltage of 12 volts results in higher rotational speed of 8,000 rpm as T2 curve shown in FIG. 2. Higher rotational speed means increasing current and the phenomenon of burned-out can occur easily if the increased current exceeds an acceptable range of current, which is capable of being endured with the driving element 121.